The physical downsizing of microprocessor-based technologies has led to the development of portable personal computers, pocket secretaries, wireless phones, and pagers. All of these devices, and other devices such as clocks, watches, calculators, etc., have the common need for a data display screen having low power consumption in order to extend the useful working period between battery replacements or battery charging.
The most common type of display in such devices is the liquid crystal display (LCD). LCDs may be classified based upon the source of illumination. Reflective displays are illuminated by ambient light entering the display from the front. A reflective surface, such as a brushed metal reflector placed behind the LCD returns light that has been transmitted through the LCD while preserving the polarization orientation of the light incident on the reflective surface. Although reflective displays meet the need for low power consumption, the displays are only useful under good ambient lighting conditions. Under low levels of ambient light, the display often appears dark and is difficult to read. Therefore, the purely reflective display has limited utility.
Another type of LCD display is a backlit display, in which backlight is used to illuminate the display. The viewer views light that has been transmitted through the LCD display from the backlight. Typically, the backlight assembly includes a lamp, light-emitting diode (LED) or other device that emits light, and some optical elements to direct the light from the light emitter to the LCD. Backlighting may also be used to supplement a reflective display so that it can be used over a wide range of ambient light conditions. However, the introduction of a backlit assembly increases the power drain on the battery, significantly reducing the useful length of a battery, or the time between battery chargings.
The combination of backlighting with an ambient reflective display introduces the need for a “transflective” film. The transflective film is placed between the LCD and the light source, and is used for reflecting ambient light that has been transmitted through the LCD and transmitting light from the light source to illuminate the LCD. However, under ambient lighting conditions, the characters displayed on the screen may be dark while the background is light, while under backlighting conditions the characters appear to be light on a dark background. In other words, the backlit image is inverted relative to the ambient reflective image. This problem may be overcome electronically, by inverting the parity of the LCD display when the backlight is illuminated. The user may experience some annoyance or discomfort when the parity of the display flips from one state to another. Another problem with this type of display is washout, which occurs when the ambient light and the backlight have approximately the same intensity, thus making it difficult to see the information presented on the display.
Furthermore, there are certain applications, such as cell phones and pagers, where it may be desirable for the backlight to be illuminated whenever the user is using the device, in order to ensure a certain minimum viewability, irrespective of the ambient lighting conditions. In such a case, it is not possible to switch the parity of the LCD, since the display has, at all times, to be able to operate in a reflective mode. Therefore, the transflective film has to permit the backlit image to be non-inverted relative to the ambient reflective image. The currently available non-inverting transflective films operate with high losses, reducing their suitability for reflecting ambient light, and increasing the amount of optical power that needs to be generated in order for the viewer to correctly view the backlit image.
Accordingly, there is a need for an improved non-inverting transflective film that will reduce the power requirements for the backlight illuminator, while maintaining high viewability of the displayed image.